The hypothesis is investigated that female sexual interest is stimulated by the neural actions of ovarian estrogens and that androgens regulate the bioavailability of these estrogens through interactions with sex hormone binding globulin (SHBG). Three projects, using a rhesus monkey model of endocrine function and behavior, investigate the hormonal basis of female sexual initiation. Project I investigates sexual initiation in females across the menstrual cycle, comparing the occurrence of female sexual initiation in a social group context during normal cycles treated with an androgen receptor blocker (flutamide) or an estrogen receptor blocker (tamoxifen). This will clarify whether androgens or estrogens act neurally to modulate female sexual motivation. Project II tests the novel hypothesis that SHBG regulates bioavailable estrogens and androgens through these steroids' different affinities for SHBG. This project uses a monkey model of hormonal replacement therapy for reproductively prime females after surgical removal of their ovaries and tests the hypothesis that chronic estradiol (E2) ceases to effectively stimulate female sexual interest as estrogen is sequestered by SHBG. It further investigates whether an androgen, 5a-dihydrotestosterone (DHT), with a markedly higher affinity for SHBG than estradiol can acutely and rapidly reinstate female sexual interest by increasing free estradiol by displacing SHBG-bound estradiol. Ovariectomized females receiving chronic estradiol treatment mimicking mid-follicular estradiol levels will be observed for sexual initiation during chronic E2 treatment alone and following chronic E2 and an injection of DHT or E2. Concurrent administration of flutamide or tamoxifen with the estrogen or DHT will discriminate between behavioral changes resulting from the activation of neural androgen or estrogen receptors. The effects of these treatments on neuroendocrine function will also be investigated. Project III investigates whether common human hormonal replacement therapies of chronic estrogen, or chronic estrogen plus testosterone with or without concurrent progestin, can reinstate female sexual interest in reproductively prime ovariectomized female monkeys. The hypothesis will be tested that chronic progestin therapy reduces or eliminates the effectiveness of therapies that reinstate female sexual interest without progestin. These therapies will also be compared on their effects on neuroendocrine function. These studies will markedly increase our understanding of the role that ovarian steroids play in modulating women's sexuality.